Eurasian Cold Extreme in January 2012: Sudden Intensification of Ural Blocking by Stratospheric Coupling

Climatic anomalies and related mechanisms for the extreme European cold winter in early 2012 are investigated. Ural blocking, a tall high-pressure system centered over Ural mountain region had developed in early January 2012 and occupied the region until mid February. The event was exceptional in the sense that the persistent high anomaly blocking was biggest at least since 1950. At the same period, central and Eastern Europe suffered extreme cold events, which resulted in over 600 casualties during the period. In this study, we show that the abrupt intensification of the Ural blocking in mid January is closely associated with the sudden breakdown of the Stratospheric Polar Vortex (SPV) and rapid adjustment of the tropospheric circulation showing the downward coupling feature. Also, it is shown that the precursor signal for the weakening of the SPV was in the troposphere and again related with the earlier development of the Ural blocking in early January. This was revealed to because the occurrence of the linear interference between climatological stationary of wavenumber 1 & 2 at that time and wavenumber 1 &2 features associated with the Ural blocking. Therefore, we suggest that development of Ural blocking and associated downstream horizontal wave propagation to Pacific provided a favorable condition for the upward propagation of Rossby wave that breaks polar vortex, which, in turn, the weakened polar vortex reinforces the Ural blocking.